Engineering Geology 157 (2013) 33–38

Contents lists available at SciVerse ScienceDirect

Engineering Geology

j ourna l homepage: www.e lsev ie r .com/ locate /enggeo

Technical Note

Influence of humidity conditions on shear strength of clay rock discontinuities

F.L. Pellet a,⁎, M. Keshavarz b,c, M. Boulon b

a INSA — University of Lyon, Department of Civil and Environmental Engineering, Villeurbanne, Franceb University of Grenoble, Laboratory 3SR, Grenoble, Francec University of Payam Nour, Faculty of Science, Zanjan, Iran

⁎ Corresponding author. Tel.: +33 683229945.E-mail addresses: frederic.pellet@cfmr-roches.org, fr

0013-7952/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.enggeo.2013.02.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 September 2012Received in revised form 20 January 2013Accepted 5 February 2013Available online 14 February 2013

Keywords:Rock discontinuityClay infill materialDirect shear testWater saturationShear strength reductionLubrication

The shear strength of rock discontinuities strongly depends on the water content especially when the rockscontain clay materials. To assess the decrease in the mechanical properties of clay-infilled discontinuities dueto water saturation, a series of direct shear tests was performed using an advanced shear box that allows theinjection of water into the discontinuity. Results show that both the friction coefficient and the cohesion de-crease when the discontinuity is saturated. Overall, the shear strength of the discontinuity is considerably re-duced to approximately 50% of its original value. This reduction has to be accounted for when conductingstability analyses of rock slopes, dam foundations or underground openings.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The stability of rock masses is largely affected by the humidityconditions which are, for example, responsible for landslides thatoccur after heavy rains. This is particularly true for stratified rockmasses with clay-infilled discontinuities, because most of clay mineralsare highly sensitive to water.

Beside the change of pore pressure distribution within the rockmass, an increase in the water content modifies the consistency ofthe clay and therefore its mechanical properties. Several landslideswere caused by a sudden drop in the mechanical properties of thematerial associated with an increase in the water content. This wasthe case, for example, in the catastrophic events of the Vaiont Damfailure, where a landslide caused the sudden emptying of the reser-voir (Hendron and Patton, 1987).

Despite some recent studies on this subject (Nara et al., 2011,2012; Li et al., 2012), the mechanical behavior of clay-infilled rockdiscontinuities under direct shear test has not yet been fully investi-gated. Several loading conditions may be encountered, from staticloading leading to creep (Zhang et al., 2012) to cyclic loadings that in-duce degradation of the rock discontinuities (Jafari et al., 2003). Interms of failure criteria, Indraratna et al. (2010) recently proposedan approach to account for discontinuity characteristics (roughness,thickness, etc.). However, it has to be remembered that, in order toextend laboratory results to the field, scale effects have to be takeninto account (Vallier et al., 2010; Fujii et al., 2011).

pellet@orange.fr (F.L. Pellet).

rights reserved.

In this framework, a testing program based on direct shear testshas been carried out on rock discontinuities in order to determinehow much the shear strength is changed when the water contentincreases.

2. Testing program

2.1. Rock under study

The rock under study is a marl from the Dogger geological period,made up of approximately 50% clay, 30% carbonate and 20% quartz. Theclayminerals aremostly illitewith some smectite and some interstratifiedminerals. The natural water content of the rock specimens lies between 4and 7% (Fabre and Pellet, 2006).

The main average physical and mechanical properties of this rock,reported in Table 1, were determined in previous studies, includingsonic velocity measurements (Pellet and Fabre, 2007). Time depen-dent properties, shrinkage and swelling characteristics of intact rockspecimens as well as hydro-mechanical properties and permeabilityhave been also extensively investigated (Buzzi et al., 2007; Cariou etal., 2009).

2.2. Testing equipment and specimen preparation

Tests were carried out using a 3D shear box developed by Boulon(1995) in order to control stresses and displacements in the threespatial directions. The originality of this device is to make shearingpossible by moving the two half boxes in opposite directions. Thisallows the normal stress to remain centered on the discontinuity,

Table 1Average values of mechanical and physical properties of the marl under study.After Fabre and Pellet (2006) and Pellet and Fabre (2007).

DensitykN/m3

Water content%

Porosity%

Saturation degree%

P-wave velocitym/s

UCSMPa

23.7 5.9 15.5 87 4200 26

Fig. 2. Specimen preparation: marl specimen sealed with a mortar in one half box (toppicture); the two half box assemblies ready to be installed in the shear box (bottompicture); dimensions of the boxes are 140 mm×140 mm.

34 F.L. Pellet et al. / Engineering Geology 157 (2013) 33–38

therefore avoiding half box rotation. The tests can be performed witha Constant Normal Load or by imposing a Constant Volume with nojoint dilation and no joint compaction. This equipment also allowsthe hydro-mechanical behavior of rock discontinuities to be studiedusing fluid injection into the center of the specimen. An extensive de-scription was given by Hans and Boulon (2003) and by Buzzi et al.(2007). The overall dimensions of this equipment, which is shownin Fig. 1, are 1.2 m×1.2 m.

The specimens used were marl cores retrieved from borehole.They were 79 mm in diameter and 80 mm in height. These specimenswere then sawed in half; consequently the artificially created disconti-nuities were perfectly smooth. Afterwards, the two parts of the speci-men were sealed in the steel half boxes using a cement mortar madeup of a mixture of fast-curing cement and Vicat ordinary cement pre-pared with an optimal ratio to obtain maximum strength for a mini-mum curing time. Fig. 2 shows the two half boxes before and afterassembly, ready to be tested. The external dimensions of a half boxare 140 mm×140 mm.

2.3. Testing program

The main part of the testing program was the direct shear teststhat were performed on the marl discontinuities in parallel to thejoint surface. Additionally, in order to fully characterize the rock dis-continuities under study, a few swelling tests and some compressiontests were performed prior to the direct shear test.

Direct shear tests were carried out under two different humidityconditions. The first series of tests were conducted on dry discontinu-ities and the second on discontinuities that had been saturated withwater. The objective of these tests was to determine the effect ofthe humidity conditions on the mechanical properties including thefriction coefficient and the cohesion of the discontinuities.

Direct shear tests were carried out with a shearing rate of 0.05 mm/s.They were performed with either a Constant Normal Load (CNL) or at aConstant Volume (CV). Both types of tests are able to determine if the

Place for boxes

Fig. 1. Photograph of the shearing equipment (BCR3D).After Hans and Boulon (2003), reproduced with the permission of JohnWiley Ltd.

discontinuity tends to dilate or to contract during shearing. The testingconditions of the performed tests are summarized in Table 2.

3. Test results and discussion

3.1. Swelling test

To measure the swelling of the marl discontinuities, water wasinjected into the center of the specimen after having applied a littlenormal stress of about 0.1 MPa. Water injections were performedtwice a day, 5 to 12 h apart. During the test, the normal displacementwas kept at almost zero. Fig. 3 shows results for test TW-CV-21, wherethe displacement was controlled and the increase of normal stress withrespect to time was recorded. After three days of water injection(4300 min), the normal stress stabilized to 0.58 MPa.We can thereforeconclude that this clay is moderately susceptible to swelling due to the

Table 2Shear test program and testing conditions: Constant Normal Load (CNL) and ConstantVolume (CV).

Test Joint conditions Loading conditions Stages of initial normal stress[MPa]

TD-CV-02 Dry CV 2TD-CNL-03 Dry CNL 2, 5, 10, 16TD-CNL-04 Dry CNL 5TD-CV-05 Dry CV 5TD-CV-06 Dry CV 16TD-CV-17 Dry CV 5, 10, 12TW-CNL-07 Saturated CNL 2, 5, 10,16TW-CV-21 Saturated CV 5, 10, 12TW-CV-23 Saturated CV 2, 5, 10

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0

0.1

0.2

0.3

0.4

0.5

0.6

0.1 1 10 100 1000 10000

Time [minutes]

Normal stress

Swelling > 0

Water injection

Sw

ellin

g [m

m]

Nor

mal

str

ess

[MP

a]

Fig. 3. Normal stress as a function of time for a displacement-controlled swelling test(test TW-CV-21).

Table 3Normal stiffness Kn [MPa/mm] for different normal stresses (2, 5, 10, 16 MPa) deter-mined on the unloading path for dry and saturated discontinuities.

2 MPa 5 MPa 10 MPa 12 MPa 16 MPa

Dry discontinuity TD-CV-02 10.8 14.0 18.2 – 22.8TD-CNL-03 14.3 – – – –

TD-CNL-04 11.9 – – –

Mean value 12.3 14.0 18.2 – 22.8Standard. dev. 1.8 – – – –

Saturateddiscontinuity

TW-CNL-07 18.1 13.9 12.1 – –

TW-CV-21 14.8 10.1 8.7 6.4 –

Mean value 16.5 12.0 10.4 6.4 –

Standard. dev. 2.3 2.7 2.4 – –

35F.L. Pellet et al. / Engineering Geology 157 (2013) 33–38

presence of smectite, as it was previously demonstrated by Sato et al.(1992).

3.2. Compression test

Fig. 4 shows the results of compression tests. Test TD-CV-02 wasperformed with four different levels of normal stress (2, 5, 10 and16 MPa) and the results are shown in Fig. 4a. After the first loading–unloading cycle (2 MPa), an irreversible displacement of about 0.2 mmwas measured. The subsequent cycles lead to a larger irreversibledisplacement, up to 1.2 mm for 16 MPa. Results for the saturated

0

2

4

6

8

10

12

14

16

18

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

Normal displacement [mm]

a

0

2

4

6

8

10

12

14

16

18

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

Normal displacement [mm]

b

Nor

mal

e st

ress

[MP

a]N

orm

al s

tres

s [M

Pa]

Fig. 4. Normal stress versus normal displacement for compression tests performedprior to the shear test, a — dry discontinuity (test TD-CV-02), b — saturated interface(TW-CV-21).

discontinuity test TW-CV-21 are presented in Fig. 4b; the values ofthe normal stiffness are less, as the rock material is softer due tothe presence of water.

The secant normal stiffness kn of the discontinuities are summarizedin Table 3. They were computed on the unloading path of each cycle.This indicates that for each stress level, the normal stiffness modulusis the difference between the maximum normal stress attained andzero (complete unloading) divided by the corresponding displacementincrement. Despite the fact that the values of the moduli are slightlyscattered, it can be seen that the normal stiffness substantially increaseswith normal stress for the dry discontinuity. On the contrary, for the sat-urated discontinuity there is a slight decrease in stiffness.

These opposite trends can be explained by the fact that although thediscontinuities were considered to be smooth from a rock engineeringpoint of view, the specimen sawing left tiny marks rather likemicro as-perities. Therefore, the surface of the dry discontinuity is less even and a

a

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-2 -1 0 1 2 3 4 5 6 7 8

Shear displacement [mm]

Βn = 2MPa

Βn = 5MPa

Βn = 10MPa

Βn = 16MPa

b

Nor

mal

dis

plac

emen

t [m

m]

-6

-4

-2

0

2

4

6

8

-2 -1 0 1 2 3 4 5 6 7 8

She

ar S

tres

s [M

Pa]

Shear displacement [mm]

σn = 2MPa

σn = 16MPa

σn = 10MPa

σn = 5MPa

Fig. 5. Shear test TD-CNL-03 performed on a dry discontinuity under Constant NormalLoad (2, 5, 10 and 16 MPa): a — Shear stress versus shear displacement, b — normaldisplacement versus shear displacement.

Table 4Shear stiffness Ks [MPa/mm] and maximum shear stress τpeak [MPa] of dry discontinu-ities for different normal stresses (2, 5, 10, 12, 16 MPa).

2 MPa 5 MPa 10 MPa 12 MPa 16 MPa

ks τpeak ks τpeak ks τpeak ks ks τpeak

TD-CV-02 6.1 0.8 – – – – – – – –

TD-CNL-03 7.2 1.0 5.9 2.4 12.7 4.2 – – 8.2 6.2TD-CNL-04 – – 3.2 2.3 – – – – – –

TD-CV-05 – – 4.8 2.9 – – – – – –

TD-CV-06 – – – – – – – – 5.8 6.9TD-CV-17 – – 4.7 2.6 5.8 5.3 9.2 6.1 – –

Mean value 6.6 0.9 4.6 2.6 9.2 4,7 9.2 6.5 7.0 6.6Standarddeviation

±0.7 ±0.1 ±1.1 ±0.2 ±4.8 ±0.8 – – ±2.3 ±0.5

a

-0.2

0.0

0 1 2 3 4 5 6 7

b

-8

-6

-4

-2

0

2

4

6

8

She

ar s

tres

s [M

Pa]

Shear displacement [mm]

σno =10MPa

σno =12MPa

σno =5 MPa

36 F.L. Pellet et al. / Engineering Geology 157 (2013) 33–38

higher normal stress is required to ensure perfect contact. On the otherhand, for saturated discontinuities, the contact between the twoparts ofthe specimen is perfect, evenunder a lownormal stress, because, as pre-viously stated, the rock material is softer.

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

0 1 2 3 4 5 6 7

Shear displacement [mm]

6 no =10MPa

6 no =12MPa

6 no =5 MPa

Nor

mal

dis

plac

emen

t [m

m]

Fig. 7. Shear test TD-CV-17 performed on a dry discontinuity with Constant Volume(initial normal stress: 5, 10, and 12 MPa): a — Shear stress versus shear displacement,b — normal displacement versus shear displacement.

3.3. Direct shear test on a dry marl discontinuity

Six direct shear tests were conducted on a dry marl discontinuity.The shear test results giving the shear stress versus shear displacementand shear displacement versus normal displacement are shown inFigs. 5 and 7. The test results are summarized in Table 4.

For an applied normal stress, the shear displacement is increaseduntil the maximum shear stress is reached. Then, the normal stressis increased to the next level and the shearing process is resumed.At the end of the actuator stroke, the shearing direction is reversedso that shearing is run in the opposite direction. At this stage, someslight decays may be observed due to the backlash compensationthat occurs when the shearing direction is changed.

Fig. 5 shows the results of Test TD-CNL-03 performed under Con-stant Normal Load conditions (CNL). The shear tests were performedat different levels of constant normal stress ranging from a minimumstress of 2 MPa to a maximum stress of 16 MPa. During the shearing,the shear stress increases with the shear displacement to reach amaximum value of 6.1 MPa and then remains stable (Figure 5a). Avery slight change in vertical displacement is observed during the shear-ing (Figure 5b) which indicates a tiny amount of contraction. However,during the reverse path, the joint tends to dilate due to gouge production.A slight decrease in the shear stress is observed, which confirms the dis-continuity damage.

By analyzing the shear stress–shear displacement curve, the shearstiffness can be determined. Their mean values lies between 4.6 and9.2 MPa/mm, but no clear dependency on the normal applied stresshas been evidenced.

Fig. 6. Photos of the dry discontinuity after testing (test TD-CNL-03). Left: lower part;right: upper part.

Photographs of the dry surface discontinuity after testing arepresented in Fig. 6. Gouge (little pieces of rock) is clearly visible indicat-ing that joint damage is responsible for the joint contraction–dilation.

Fig. 7 shows the curves for the test TD-CV-17 carried out underConstant Volume conditions (CV) with an initial normal stress of 5,10 and 12 MPa. The shear stress increases with the shear displace-ment (Figure 7a) whereas the normal displacement remains constant(Figure 7b). In Fig. 8, it is shown that the normal stress tends to decreaseduring the shearing process, for both the forward path and the reversepath. This clearly indicates a decrease in the shear strength due to dis-continuity damage.

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7

Shear displacement [mm]

Nor

mal

str

ess

[MP

a]

Fig. 8. Normal stress versus shear displacement — test TD-CV-17 performed on a drydiscontinuity with Constant Volume (initial normal stress: 5, 10, and 12 MPa).

Table 5Shear stiffness Ks [MPa/mm] and maximum shear stress τpeak [MPa] of saturated dis-continuities for different normal stresses (2, 5, 10, 12, 16 MPa).

2 MPa 5 MPa 10 MPa 12 MPa 16 MPa

ks τpeak ks τpeak ks τpeak ks τpeak ks τpeak

TW-CNL-07 15.0 0.9 14.1 1.5 12.0 2.3 – – 9.1 4.4TW-CV-21 – – 7.6 1.5 4.5 2.2 3.0 2.4 – –

TW-CV-23 2.6 0.8 4.1 1.4 5.1 2.4 – – – –

Mean value 9.0 0.8 7.6 1.5 7.3 2.2 3.0 2.4 9.1 4.4Standarddeviation

±8.7 ±0.1 ±3.5 ±0.1 ±1.2 ±0.1 – – – –

Fig. 10. Photos of the discontinuity after testing (test TD-CNL-07). Left: lower part;right: upper part.

37F.L. Pellet et al. / Engineering Geology 157 (2013) 33–38

3.4. Direct shear test on a saturated marl discontinuity

Three tests were performed on saturated marl discontinuities(Table 5). Prior to testing, water is injected over three days while thediscontinuity is subjected to a small normal stress (0.1 MPa). The evolu-tion of the normal stress with respect to time was shown in Fig. 3 fortest TW-CV-21.

Fig. 9a shows the shear stress–shear displacement curves for a testperformed at Constant Normal Load (Test TW-CNL-07). The resultsshow almost the same shape than those obtained under dry condi-tions but the maximum shear stress is considerably reduced. In thereverse path the normal stress was lowered from 16 MPa to 8 MPa,in order to avoid excessive damage to the discontinuity. This leads toa drop in the shear stress for a shear displacement of about 4.3 mm.

Fig. 9b, shows the normal versus the shear displacement curve. Itis clearly evident that when the sample is saturated, the discontinuitycontraction is muchmore important at high normal stresses due to thehigher compressibility of the saturated claymaterial. The shear stiffnesshas approximately the same magnitude as that observed in the drydiscontinuities.

0 1 2 3 4 5 6 7 8

She

ar s

tres

s [M

Pa]

Shear displacement [mm

σn= 10 MPa

σn= 16 MPa

σn= 5 MPa

σn= 2 MPa

-2 -1 0 1 2 3 4 5 6 7 8

Nor

mal

dis

plac

emen

t [m

m]

Shear displacement [mm]

σn= 10 MPa

σn= 16 MPa

σn= 5 MPa

σn= 2 MPa

-8

-6

-4

-2

0

2

4

6

-1

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

a

b

Fig. 9. Shear test TW-CNL-07 performed on a saturated discontinuity with initial nor-mal stresses of 5, 10, and 16 MPa: a— Shear stress versus shear displacement, b— normaldisplacement versus shear displacement.

Thephotographs in Fig. 10 show the saturatedmarl discontinuity aftertesting. Damage ismore severe than in the dry condition (Figure 6). In ad-dition to solid debris (gouge), some muddy areas are clearly visible.

When shearing in the opposite direction (reverse path) this gougematerial is rearranged; this can lead to extra dilation or contraction ofthe discontinuity (Figure 9b). In conclusion, for a saturated disconti-nuity, the reverse path provides less reliable results.

The results of a test performed under Constant Volume conditions(TW-CV-23) are shown in Fig. 11. The shear stress increases withshear displacement as the normal displacement is kept constant dur-ing the shearing process. For the last stage of shearing (when the nor-mal stress reaches 10 MPa) the shear stress drops due to damage tothe discontinuity and rock matrix. In Fig. 12, a significant decreasein the normal stress is observed. This observation is consistent withthe results of test TW-CNL-07where a substantial contraction of the dis-continuity was observed. In the present test (TW-CV-23), the normalstress is adjusted to prevent compaction (Constant Volume conditions).

a

b

-4

-2

0

2

4

-1 0 1 2 3 4 5 6 7 8 9

She

ar s

tres

s [M

Pa]

Shear displacement [mm]

= 5 MPa

= 10 MPa

= 2 MPaσ

no= 1 MPa

-2.0

-1.5

-1.0

-0.5

0.0

-1 0 1 2 3 4 5 6 7 8 9

Nor

mal

dis

plac

emen

t [m

m]

Shear displacement [mm]

= 5 MPa

= 10 MPa

= 2 MPa

= 1 MPa

σno

σno

σno

σno

σno

σno

σno

Fig. 11. Shear test TW-CV-23 (Constant Volume) performed on a saturated discontinu-ity with initial normal stresses of 2, 5, and 10 MPa: a — Shear stress versus shear dis-placement, b — normal displacement versus shear displacement.

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9

Nor

mal

str

ess

[MP

a]

Shear Displacement [mm]

Fig. 12. Normal stress versus shear displacement — test TW-CV-23 performed on satu-rated discontinuity with Constant Volume (initial normal stress: 2, 5, and 10 MPa).

38 F.L. Pellet et al. / Engineering Geology 157 (2013) 33–38

4. Discussion on the effect of water content on the shear properties

The observations described above outline a significant differencebetween the behavior of dry and saturated discontinuities.

In terms of shear strength, the maximum shear stress versus normalstress is presented in Fig. 13a for dry discontinuities and in Fig. 13b forsaturated discontinuities. In both cases, it seems that theMohr–Coulombfailure criterion is relevant, at least up to a normal stress of 10 MPa. Dueto the lubricant effect of water, the friction angle is much lower for thesaturated discontinuities with a substantial drop from 22° for a dry dis-continuity to 12° for the saturated discontinuity. At the same time cohe-sion slightly decreases from 0.41 MPa to 0.32 MPa. Additionally, bothnormal and shear stiffness moduli change considerably in the presenceof water.

a

b

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12 14 16 18

Normal stress [MPa]

Mohr-Coulomb

TD-CNL-03

TD-CNL-04

TD-CV-06

TD-CV-02

TD-CV-17

TD-CV-05

Cohesion = 0.41 MPaFriction angle= 22°

Max

imum

she

ar s

tres

s [M

Pa]

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12 14 16 18

Normal stress [MPa]

Mohr-Coulomb

TW-CNL-07

TW-CV-21

TW-CV-23

Max

imum

she

ar s

tres

s [M

Pa]

Cohesion = 0.32 MPaFriction angle = 12 °

Fig. 13. Normal stress versusmaximumshear stress, a— dry discontinuities, b— saturateddiscontinuities.

Despite the fact that additional tests would be needed to investi-gate the effect of different water content and saturation ratios, thestrength reduction of the discontinuities, evidenced before, explainswhy the rock slope stability could be critical during the rainy seasonor after the snow melting period. Moreover, the changes in stiffnessmoduli enhance the acceleration of movements recorded in rock slopesthat are monitored. In this respect, some interesting correlations couldbe achieved on case histories.

5. Conclusions

The mechanical properties of infilled-clay discontinuities are sub-stantially changed by the humidity conditions (i.e. water content).Overall, the shear strength is considerably reduced when the watercontent increases. Even if this conclusion is not surprising, it is impor-tant to accurately determine howmuch the strength of the material isreduced when its water content is increased. In the present study, theshear strength of a dry discontinuity in a marl specimen is reduced byapproximately 50% when the discontinuity is saturated. This conclu-sion should help when performing stability analyses of rock slopes,dam foundations or underground openings by taking into accountmore realistic discontinuity shear strength parameters. Therefore inthe future, safer and more reliable designs could be achieved.

References

Boulon, M., 1995. A 3D direct shear device for testing the mechanical behaviour and thehydraulic conductivity of rock joints. Second Int. Conf. on Mechanics of Jointed andFaulted Rock MJFR-2, Vienne, Balkema ed. Rotterdam, Pays Bas, pp 407-413.

Buzzi, O., Hans, J., Boulon, M., Deleruyelle, F., Besnus, F., 2007. Hydromechanical studyof rock–mortar interfaces. Physics and Chemistry of the Earth 32, 820–831.

Cariou, S., Dormieux, L., Skoczylas, F., 2009. Swelling of a bentonite plug: amicrocmechanicalapproach. In: Eberhardsteiner, J., et al. (Ed.), ECCOMASMultidisciplinary Jubilee Sympo-sium, Computational Methods in Applied Sciences.

Fabre, G., Pellet, F., 2006. Creep and time dependent damage in argillaceous rocks. In-ternational Journal of Rock Mechanics and Mining Sciences 43, 950–960.

Fujii, Y., Ishijima, Y., Ichihara, Y., Kiyama, T., Kumakura, S., Takada, M., Sugawara, T.,Narita, T., Kodama, J.I., Sawada, M., Nakata, E., 2011. Mechanical properties of aban-doned and closed roadways in the Kushiro Coal Mine — Japan. International Jour-nal of Rock Mechanics and Mining Sciences 48, 585–596.

Hans, J., Boulon, M., 2003. A new device for investigating the hydro-mechanical proper-ties of rock joints. International Journal for Numerical and Analytical Methods inGeomechanics 27, 513–548.

Hendron Jr., A.J., Patton, F.D., 1987. The Vaiont slide — a geotechnical analysis based onnew geologic observations of the failure surface. Engineering Geology 24, 475–491.

Indraratna, B., Oliveira, D.A.F., Brown, E.T., 2010. A shear-displacement criterion forsoil-infilled rock discontinuities. Geotechnique 60, 623–633.

Jafari, M.K., Amini Hosseini, K., Pellet, F., Boulon, M., Buzzi, O., 2003. Evaluation of shearstrength of rock joints subjected to cyclic loading. Soil Dynamics and EarthquakeEngineering 23, 619–630.

Li, D., Wong, L.N.Y., Liu, G., Zhang, X., 2012. Influence of water content and anisotropyon the strength and deformability of low porosity meta-sedimentary rocks undertriaxial compression. Engineering Geology 126, 46–66.

Nara, Y., Morimoto, K., Yoneda, T., Hiroyoshi, N., Kaneko, K., 2011. Effects of humidityand temperature on subcritical crack growth in sandstone. International Journalof Solids and Structures 48, 1130–1140.

Nara, Y., Morimoto, K., Hiroyoshi, N., Yoneda, T., Kaneko, K., Benson, P.M., 2012. Influ-ence of relative humidity on fracture toughness of rock: implications for subcriticalcrack growth. International Journal of Solids and Structures 49, 2471–2481.

Pellet, F., Fabre, G., 2007. Damage evaluation with P-wave velocity measurements dur-ing uniaxial compression tests on argillaceous rocks. International Journal ofGeomechanics, American Society of Civil Engineers (ASCE) 7, 431–436.

Sato, T., Watanabe, T., Otsuka, R., 1992. Effects of layer charge, charge location, and energychange on expansion properties of dioctahedral smectites. Clays and Clay Minerals40, 103–113.

Vallier, F., Mitani, Y., Boulon, M., Esaki, T., Pellet, F., 2010. A new shear model for accountingscale effect in rock joints behaviour. RockMechanics and Rock Engineering 43, 581–595.

Zhang, Q., Shen, M., Ding, W., Clark, C., 2012. Shearing creep properties of cements withdifferent irregularities on two surfaces. Journal of Geophysics and Engineering 9,210–217.